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Abstract

Three-dimensional classical ensembles are employed to study recollision dynamics
in double ionization of atoms by 780-nm intense lasers. After recollision one
electron typically remains bound to the atom for a portion of a laser cycle,
during which time the nucleus strongly influences its direction of motion. The
electron then escapes over a suppressed barrier, with its final momentum
depending critically on the laser phase at escape. The other electron remains
unbound after collision, and typically drifts out in a momentum hemisphere
opposite from its motion just after the collision. Several example trajectories
at intensity 0.4 PW/cm2 with various time delays between recollision
and ionization are presented.

Figures (9)

Scatter plots of final momentum components along the laser polarization axis
for laser intensity 4 × 1014 W/cm2 and for
the indicated time delay intervals between rec-ollision and double
ionization. For each trajectory, p2z denotes
final z-component of the momentum of the recolliding electron, and
p1z the struck electron. The signs of
the final momenta are defined so that all recollision events occur with the
recolliding electron having p2z > 0.
The boxes show (4Up)1/2, with
Up=0.838 au. For time delays less than
0.25 cycle, most electrons emerge in the momentum hemisphere opposite from
recollision, so pz < 0 in the
plot.

Percent of doubly ionizing trajectories vs. laser phase for recollision (left
column) and double ionization (right column) for laser intensity
4×1014 W/cm2. The red and green
respectively show the same-hemisphere and opposite-hemisphere trajectories
for various maximum time lags, with blue giving all remaining DI
trajectories. The top three rows show time delays of less than 1/25 cycle,
1/4 cycle, and 1/2 cycle respectively. The fourth row classifies all
trajectories as same- or opposite-hemisphere regardless of time delay. The
phase difference between recollision and double ionization is clearly
evident.

Distribution of time delays for four different laser intensities. Red and
green indicate same-hemisphere and opposite-hemisphere trajectories,
respectively. Plots only extend to a delay time of 1 cycle, but there are
scattered delay times up to 6.9 cycles. The percent of DI trajectories with
delay times of one cycle or less are 86%, 88%, 90%, and 89%, for the
respective laser intensities. Total yields for the four intensities were
1721, 3503, 4320, and 4927 trajectories of 400,000.

On the left is a movie (64 kB) of one two-electron trajectory that exhibits
direct recollision ionization for laser intensity I = 0.4 PW/cm2.
There is a change in direction of the electrons after recollision. The still
shows time 3.90 c with both electrons traveling outward after ionization. On
the right is an energy vs time plot for the two electrons. The energy
transfer at recollision is clearly visible in the inset. [Media 1]

Movies (72 and 244 kB)of the trajectory of Fig. 5. The right plot shows the z-part of the
motion, with effective potential energy plots for each electron. The curves
have a parametric dependence on the x and y values.The still images show
times shortly after recollision when the struck (blue-coded) electron still
has energy less than zero. [Media 2] [Media 3]

Trajectory and effective energy movies (132 and 964 kB) for the trajectory of Fig. 7. The struck electron ionizes after the field
maximum and drifts out opposite from its initial ionization. Still images
are shortly after ionization, which by our definition occurs at 6.79 c. [Media 4] [Media 5]